Restorative dentistry is an important market in today's dental industry. In particular, tooth repair with temporary and permanent crowns is a common procedure, typically requiring multiple dental appointments. Current technology uses pastes, two-part powder/liquid systems, preformed metal temporary crowns, and ceramic or porcelain/metal permanent crowns.
Currently available polymerizable resins are typically free radically polymerizable monomers (e.g., acrylates, methacrylates, and acrylamides). These are used together with initiator systems that typically include reducing agents, such as tertiary amines, and oxidizing agents, such as peroxides, as accelerators or activators for free radical polymerization and control of the rate of polymerization.
A typical procedure for making a provisional (i.e., temporary) dental restorative involves the following steps. Initially, an alginate impression is taken before preparing the teeth. The impression is rinsed, set aside, and wrapped in a moist paper towel. The teeth are then prepared and the correct shade of acrylic powder is selected to match the natural teeth. An acrylic liquid resin and the acrylic polymeric powder, one of which includes a reducing agent and the other of which includes an oxidizing agent, are mixed together and placed in the impression. The impression is placed aside until the composition thickens and forms a dull appearance (approximately 45-60 seconds). Meanwhile, the prepared teeth and surrounding tissue are coated with a petroleum jelly, which ensures easy removal of the acrylic temporary from the preparation and protects the teeth and tissue from irritation by the acrylic mixture. The impression with the acrylic mixture is seated in the mouth and held in place for a sufficient time to allow it to harden to a removable state. The acrylic material is removed from the impression and gross excess acrylic is trimmed. The acrylic material is placed in and out of the mouth while the acrylic material is in a rubbery state. The acrylic material is removed from the mouth and set aside until the acrylic is fully cured. The fit of the acrylic restorative is checked and adapted to fit, if necessary. Excess acrylic is trimmed with an acrylic bur or stone and polished to a smooth finish. The acrylic temporary is then cemented into place.
It would be desirable to eliminate the initial mixing of the liquid resin and the polymeric powder and thereby create such prosthetic devices more efficiently. It would also be desirable to eliminate the impression-taking step. Dental waxes, commonly used for taking impressions in the mouth, exhibit many desirable properties for creating devices that are customized to a patient's mouth. These properties include malleability, low memory, sufficient strength to be self-supporting, and the thermal and rheological properties shown in FIG. 1. These wax (e.g., paraffin) materials typically have melting points near 55° C., with softening transitions near 40° C. Elastic and viscous moduli G′ and G″ are approximately 106 Pascals (Pa) at 25° C., sufficiently low to be easily deformed without being tacky. Although these materials exhibit desirable properties for creating devices customized to fit a patient's mouth, they are not hardenable (e.g., through polymerization), nor do they possess desirable properties such as compressive strength and wear resistance. As a result, these materials are not suitable for dental prosthetic applications.
U.S. Pat. No. 6,057,383 (Völkel et al.) discloses a dental material based on polymerizable waxes, wherein the materials are malleable and curable; however, they are based on little or no filler, typically 0-60% by weight, and high amounts of waxes, typically more than 20% by weight. As such, these materials have generally poor mechanical properties, such as flexural strength and wear resistance. Other thermoplastic molding compounds have been prepared, but these are typically highly viscous above their melting point (Tm), and somewhat elastic below Tm due to the high molecular weight of the included polymer. Moreover, these compositions must typically be warmed significantly above room temperature before becoming malleable.
It would be desirable to have highly filled materials that can be preformed into a desirable shape yet be sufficiently malleable, particularly at room temperature or body temperature, to form a custom-shaped device.